Andrew Goodale has a nifty library, GAJavaScript, that gives you an
Objective-C interface to invoke Javascript in your web views. It helps box and
unbox parameters crossing the bridge and lets you invoke Javascript functions
as if they were Objective-C methods.

Even if the library itself is too bulky for your needs you’d do well to browse
the code. Good examples of interfacing with web views and Objective-C
metaprogramming.